We propose to purchase a Beckman Optima XL-A Analytical Ultracentrifuge to serve as a Departmental facility in the UCSF Department of Biochemistry and Biophysics. The Department contains more than 40 active research groups, and we anticipate that this ultracentrifuge will find wide use throughout the Department. It will be placed in a shared centrifuge facility that presently houses preparative ultracentrifuges. The cost of maintaining and servicing the instrument will be recharged by the Department to the individual laboratories according to use. The instrument will be overseen by Chris Field, who has been a Staff Research Associate in the Alberts and Kirschner laboratories for more than ten years. The major users listed on the grant are Marc Kirschner, whose project is entitled "Oligomerization of Tubulins and Cell Cycle Proteins;" Bruce Alberts ("Isolation and Characterization of Multiprotein Complexes"); Tim Mitchison ("Microtubule Dynamics and Mitotic Mechanism"); and Alexander (Sandy) Johnson ("Combinatorial Control of Gene Expression in Yeast"). Both Alberts and Kirschner have made extensive use of the old Beck" Model E Analytical Ultracentrifuge in their past scientific work, and both have taught the theory and practice of hydrodynamic measurements on macromolecules in graduate school courses. The role that this type of instrument plays in biology has decreased considerably in the past dime decades. But the central issues in biology have changed in recent years. After a very productive phase in which powerful new methods have been used to identify and isolate many new proteins, we now need to focus on figuring out how these proteins work in the cell. Since most proteins function in protein complexes, we need powerful ways of deciphering macromolecular interactions. Analytical ultracentrifugation is an invaluable technique for determining native molecular weights, and it is also a powerful way of measuring conformational changes, densities, macromolecule heterogeneity, and ligand binding to proteins. At UCSF, the environment in which the new ultracentrifuge will be kept will make it available to a large community. Moreover, to make students, postdoctoral fellows, and faculty aware of the potential of this instrument, hydrodynamic theory will be taught in our required Structural Biology course for first year graduate students. In addition, we will include descriptions of advanced techniques using analytical ultracentrifugation as part of our on-going Special Topics Course taught on Saturdays.